This invention relates to a method and apparatus for charging waterlogged boreholes with water-resistant particulate explosive material.
The use of commercial explosives is wide-spread. They are used in mining operations and the construction industry, for example, for blasting out foundation holes and digging trenches for pipe laying.
A common blasting technique is to sink drill holes 21/4 to 157/8 inches in diameter, depending on the application, in five hole arrays arranged with four of the holes forming the corners of a four foot square and the fifth hole in the centre of the array. The holes can be anything from 20 to 60 feet or more deep, again depending on the application.
Once the holes have been drilled they are packed with explosives. In this context there are three main types of explosives:
1. Cardboard packed dynamite type products, which cost about $250.00 per 100 kilograms
2. Plastic wrapped products, which cost in excess of $400.00 per 100 kilograms
3. Poured granular material, which costs anything from $60.00 to $160.00 per 100 kilograms.
The first two types are referred to as packaged explosives. The preferred technique is to use poured granular material. In this technique a detonator is placed at the bottom of the borehole, and the granular material is subsequently poured in until the borehole is full. The advantage is two-fold: Firstly the granular material is much cheaper than the other material. Secondly, imperfect contact between the wall of the hole and the explosive charge gives rise to an effect known as "decoupling". This effect, caused by the gap between the wall of the hole and the explosive charge, reduces the power of the explosion by interfering with the transmission of the shock wave into the surrounding rock strata.
Also, it is much easier to fill a hole with granular explosives because the material can be supplied from a storage tank mounted on a truck.
While it is desirable to use granular explosives, it has not been hitherto possible to use this material in water-bearing rock. While coated water-resistant granular explosive material is available, it cannot be packed into a waterlogged borehole because the water permeates between the particles of explosive and prevents detonation from occurring. In a waterlogged environment, it has therefore hitherto been necessary to employ packaged explosives of either the cardboard cartridge or plastic wrapped type. These cost from $250.00 to $400.00 per 100 kilogram, and furthermore are inconvenient to install because they have to be stacked one upon the other in the borehole.
An object of the invention is to alleviate the aforementioned problems of the prior art.